The present invention relates to a mechanism for continuously supplying a dampening medium or solution to a print master plate mounted over a plate cylinder in an offset printing machine.
One conventional mechanism for continuously supplying a dampening medium is illustrated in FIG. 1. In the conventional arrangement, there are provided an inking system A, a dampening system B, and a roller connecting and disconnecting mechanism C. The inking system A includes an ink transfer roller 22 and ink applicator rollers 14, 14' in contact with a master plate (not shown) disposed over a plate cylinder 7.
In the dampening system B, a dampening medium is contained in a dampening medium tank or fountain 1 with a dipping roller 3 placed therein. While the amount of the dampening medium supplied from the tank 1 is being regulated by a metering roller 2 held in rolling contact with the dipping roller 3, the dampening medium supplied from the tank 1 is transferred from the dipping roller 3 to a dampening medium distributor roller 4 which is movable into and out of contact with a dampening medium transfer roller 5. The dampening medium then is fed from the dampening medium distributor roller 4 through the dampening medium transfer roller 5 to an applicator roller 6. By bringing the applicator roller 6 into contact with a plate cylinder 7, the dampening medium on the applicator roller 6 is coated on the master plate. At the same time, ink which has been transferred from the inking system A through an ink distributor roller or an intermediate roller 15, which belongs to the mechanism C, to the applicator roller 6 also is applied to the master plate. The illustrated dampening system is disclosed in Japanese Patent Application Kokai No. 55-57464.
The dampening medium applicator roller 6 is made of a resilient material such as rubber for mixing the dampening medium with the ink which has been transferred or applied to the applicator roller 6. With respect to the rollers of the dampening unit A positioned upstream of the applicator roller 6, several rollers are provided with a hydrophilic metal layer such as a plated chromium, layer at outer peripheral surfaces thereof, and the remaining rollers of the dampening system A are provided with an elastic layer such as rubber, and these two kinds of layers are provided alternately with respect to the neighboring rollers. More specifically, the transfer roller 5 positioned immediately upstream of the applicator roller 6 is provided with the hydrophilic metal layer, and the distributer roller 4 positioned immediately upstream of the roller 5 is provided with the elastic layer. Furthermore, the dipping roller 3 positioned immediately upstream of the roller 4 is provided with the hydrophilic metal layer, and the metering roller 2 positioned adjacent to the roller 3 is provided with the elastic layer.
The dampening medium transfer roller 5, which is coated with the hydrophilic metal layer, must remain hydrophilic at all times during a printing process in order to provide a uniform supply of the dampening medium to the master plate. Therefore, it is necessary to avoid an irregular dampening medium supply which would otherwise be caused by ink applied to the roller 5.
In this connection, ink adhesion to the dampening medium transfer roller 5 must be obviated during an inoperative state of the printing machine or even during an operative state thereof. For this reason, when the distributor roller 4 is displaced out of contact with the transfer roller 5 and hence no dampening medium is supplied to the transfer roller 5 irrespective of whether the printing press is in operation or not, the transfer roller 5 and the applicator roller 6 have to be separated from each other. Only after the distributor roller 4 is brought into contact with the transfer roller 6 to supply the dampening medium to the transfer roller 5 are, the transfer roller 5 and the applicator roller 6 held against each other, thus preventing any ink from being applied to the transfer roller 5.
Therefore, in such conventional arrangement, complex roller contacting and separating mechanism is required to effect such roller movements. In addition, even if the transfer roller 5 is moved into contact with the applicator roller 6 after the dampening medium has been supplied to the transfer roller 5, ink coated on the original plate on the plate cylinder 7 in a printing process may be applied through the elastic applicator roller 6 to the transfer roller 5. When this happens, the printing process must be interrupted, and the transfer roller 5 must be cleaned. Accordingly, the transfer roller 5 requires special care to make itself operable normally, and the cleaning of the transfer roller 5 lowers the efficiency of the printing process.
The applicants have proposed a device for connecting and disconnecting a dampening unit B and an inking unit A in an offset printing machine as disclosed in copending U.S. patent application Ser. No. 350,889 filed on May 12, 1989 (corresponding to Japanese patent application No. 63-117969). This proposal is based on another conventional arrangement shown in FIG. 2. According to this device, an ink distributor roller or an intermediate roller 15 is contactable with one of the rollers of the inking unit A. The inking unit A and the dampening unit B are connected to or disconnected from each other by moving the intermediate roller 15 into and out of contact with the dampening medium transfer roller 5 of the dampening unit B. Incidentally, the inking unit A includes an ink fountain roller 13, an ink ductor roller 16, an ink fast roller 17, a first ink delivery roller 18, an ink delivery transfer roller 19, a second ink delivery roller 20, a third ink delivery roller 21, an ink transfer roller 22, and ink applicator rollers 14 14'. The plate cylinder 7 is in rolling contact with a blanket cylinder 8 which is in rolling contact with an impression cylinder 9. A printing sheet 11 is fed between the blanket cylinder 8 and the impression cylinder 9 by means of a sheet feed mechanism 10, and the sheet carrying an output image is discharged onto a sheet table 12.
With this arrangement, ink is transferred by way of the intermediate roller 15 from the inking unit A to the dampening unit B. Therefore, the intermediate roller 15 may be brought into contact with the transfer roller 5, only after a dampening medium has been supplied to the transfer roller 5 and after a printing process has been started, if the roller 5 has the hydrophilic metal layer. Therefore, a desirable output image may not be obtainable promptly at an initial stage of the printing operation in the proposed device.